A low refractive index film is formed on an optical member as an optical functional film included in an anti-reflection film, a reflective film, a half-transparent half-reflective film, a visible light-reflective infrared-transparent film, an infrared-reflective visible light-transparent film, a blue reflective film, a green reflective or red reflective film, an emission line cutoff filter or a color correction film.
Without being limited to optical members having a flat surface shape, in optical functional members such as a Fresnel lens, a lenticular lens and a microlens, which are used in brightness enhancing lens films for liquid crystal backlight, diffusion films or screens for video projection televisions, desired geometric optical performances are obtained in all of such members by using resin materials having a microstructure. Even for the surfaces of these microstructures, an optical functional film including a low refractive index film is needed.
A low refractive index film serves as an anti-reflection film having a single layer structure and exhibits the anti-reflection performance in a broader range of wavelength. Furthermore, the anti-reflection film having a single layer structure brings about a reduction in cost due to a decreased number of layers as compared with an anti-reflection film having a multilayer structure. In regard to the refractive index of the anti-reflection film having a single layer structure, when the substrate is a transparent material such as a resin material, a low refractive index in the range of 1.2 to 1.3 is desirable.
Examples of the method for forming a low refractive index film include a gas phase method such as a vapor deposition method or a sputtering method, and a coating method such as a dipping method or a spin coating method.
However, a representative thin film having a low refractive index obtained by a gas phase method is an MgF2 film having a refractive index of 1.38 or an LiF film having a refractive index of 1.39, and the performance of these thin films as single layer anti-reflection films is low.
Furthermore, representative examples of the material for a low refractive index film that is obtained by a coating method include a fluoropolymer material having a refractive index of 1.35 to 1.4, and a porous material obtained by fusing microparticles formed of a polymer of a fluoromonomer having a refractive index of 1.37 to 1.46 (see, for example, Patent Document 1). However, a fluoropolymer material having a refractive index of 1.3 or less has not yet been obtained.
Meanwhile, examples of a low refractive index film which is converted from a film having a porous structure obtained by calcination, include a porous SOG film and a porous film of magnesium fluoride (see, for example, Patent Documents 2 and 3).
However, a porous SOG film requires a calcination treatment at 200° C. or higher in order to have a refractive index of 1.3 or less, while a porous film of magnesium fluoride requires a heat treatment for one hour at 150° C. Accordingly, from the viewpoint of the heat resistance of the resin material or the structure maintenance of the microstructure, a low refractive index film requiring calcination is not suitable as an anti-reflection film for the use in resin substrates.
Also in the case where a solid substrate has a microstructure such as a Fresnel lens or a lenticular lens, a low refractive index film having a single layer structure is effective in suppressing reflected light at the lens surface and suppressing the ghost of an image that is projected on a screen for video projection or the like. Furthermore, an anti-reflection film can increase the amount of transmitted light in other optical functional members as well.
In a gas phase method, a thin film can be formed to conform to the shape of a microstructure. However, since a gas phase method requires a vacuum apparatus, the production cost rises. Furthermore, the film formed on the inner walls of the vacuum apparatus peels off and falls on the low refractive index film, remaining as foreign matter. In addition, substrate heating that is commonly carried out to obtain adhesiveness of the low refractive index film, causes cracks in the microstructure formed of a resin due to thermal stress (see, for example, Patent Document 4).
A coating method does not require a vacuum apparatus, and the foreign matter originating from a vacuum apparatus is not generated.
However, in a spin coating method, the coating material cannot be prevented from remaining on the recess areas of the microstructure, and the low refractive index film at the recess areas is thickened. When the low refractive index film does not conform to the shape of the microstructure as such, the geometric optical performance such as diffusibility and light-harvesting capability attributed to the microstructure is impaired.
On the other hand, in a dip coating method or the like, the film thickness can be controlled by the lifting speed, and therefore, it is also possible to make the coating material to conform to the shape of the microstructure.
However, since it is necessary to slow down the lifting speed even to several ten micrometers per second, the production cost noticeably increases (see, for example, Patent Document 5).
As a method for forming a thin film having a thickness in a nanometer scale from a solution, an alternate lamination method has been suggested (see, for example, Non-Patent Document 1). In the alternative lamination method, a thin film is formed by electrostatic adsorption in a liquid, and therefore, a thin film formed in satisfactory conformity with the shape of the microstructure can be obtained. Furthermore, since the method is carried out by a normal temperature process, the method does not cause thermal damage to the resin material.
A thin film obtained by alternately laminating an electrolyte polymer having a positive charge and an electrolyte polymer having a negative charge, is turned into a low refractive index film having a refractive index of about 1.2 by generating voids in the thin film through a hydrochloric acid treatment (see, for example, Patent Documents 6 and 7).
On the other hand, a microparticle single layer film produced by electrostatically adsorbing one layer of microparticles on an electrolyte polymer layer, is turned into a low refractive index film without requiring an acid treatment or the like (see, for example, Patent Documents 8 and 9). The reason why a microparticle single layer film is turned into a low refractive index film is that the surface concavo-convex shape formed by microparticles having a diameter of 100 nm or more continuously changes the refractive index, and the voids between the microparticles decrease the average refractive index.
However, a microparticle single layer film using microparticles having a diameter exceeding 100 nm scatters and diffuses visible light, and is therefore inappropriate for optical members which require transparency.
Furthermore, in the case of an optical member having a microstructure on the surface, for example, when the microstructure is a lens, if the low refractive index film at the lens surface scatters and diffuses light, the low refractive index film causes a decrease in the geometric optical performance, such as inability to focus light.
Meanwhile, in the case of using microparticles having a diameter of 100 nm or less, a transparent microparticle-laminated film is likely to be obtained.
However, the refractive index decreasing effect attributable to the surface concavo-convex shape cannot be obtained. Accordingly, lowering of an average refractive index of the microparticle-laminated film is achieved by increasing the density of voids between the microparticles (see, for example, Patent Documents 10 to 12).
These microparticle-laminated films on optical members need to have adhesiveness to substrates. The microparticle-laminated films also need durability against an adhesive tape used for surface protection, prevention of contamination or fixing at the time of processing, transport, assembly and storage of optical members having microparticle-laminated films formed thereon.
On the other hand, in recent years, under the purpose of further miniaturization and an enhancement of production efficiency of camera modules used in image pickup apparatuses such as mobile phones, production is carried out such that a camera module produced by mounting an image pickup element or the like on a lens module, is packaged on a circuit board having electronic parts mounted thereon, and then the assembly is placed in a reflow furnace to be solder welded.
In this production method, all of the constituent parts need to have heat resistance performance at the reflow temperature.
Furthermore, it is preferable for an image pickup lens module which uses an image pickup element such as CCD or CMOS and is incorporated into a mobile phone or the like, to have staunch reproducibility of object.
Recently, image pickup elements have been miniaturized, and concomitantly with this, the demand for miniaturization and compactization of image pickup lenses that are incorporated into the image pickup elements, is also inevitably increasing.
Additionally, image pickup elements are being produced to have a larger number of pixels in the order of megapixels, while image pickup lens modules using these image pickup elements are demanded to be inexpensive and lightweight.
However, in a small-sized image pickup lens module using a transparent plastic lens, there is a risk that the lens module cannot withstand the reflow temperature reaching up to 260° C. or higher and may be deformed.
Therefore, packaging of an image pickup lens module on a substrate is provided, through the use of connectors and the like, in a separate process after a solder reflow process.
Therefore, the cost for connectors and the cost for installation process are inevitable, and even from the viewpoint of lowering the cost, there is a demand for a small-sized image pickup lens module that can withstand the solder reflow temperature.
Particularly, plastics are vulnerable to heat and are prone to undergo expansion or deformation, and many of them have a problem in the deterioration of quality caused by these properties.
Furthermore, a metal oxide thin film formed by vacuum deposition or the like, or an anti-reflection film based on a metal fluorine compound has a very small coefficient of thermal expansion compared with plastics. Therefore, such an anti-reflection film is prone to have cracks (fissure) that are attributed to the heat-induced expansion and deformation of the plastic that serves as a substrate, and there is a possibility that cracks may occur at temperatures that can be encountered in the daily life, for example, at a site exposed to direct sunlight, inside a car, or in a bathroom.
As a technology previously disclosed in order to solve these problems, Patent Document 13 describes a method for producing an anti-reflection film according to a method of plasma polymerization of an organometallic compound.
Patent Document 14 also describes a method of using a hydrolysis product of an organosilane in an anti-reflection film.